U.S. Pat. No. 3,160,510 discloses quinacridonequinone as one component of a solid solution of two or more quinacridone derivatives. Such solid solutions are said to possess enhanced light fastness and tinctorial values. However, as compared to compositions containing quinacridone pigments, compositions containing quinacridonequinone pigments display poor light fastness. U.S. Pat. Nos. 3,341,345 and 3,748,162 disclose colorless quinacridonequinone stabilizers such as N,N'-diphenyl-p-phenylenediamine and 6,13-dihydroquinacridone.
U.S. Ser. No. 969,533, filed Dec. 14, 1978, now abandoned discloses 2-(phenyl)amino-9(10H)acridinone, also named 2-anilinoacridone and 1,2,3,4-tetrahydro-7-(phenyl)amino-9(10H)acridinone, also named 5,6,7,8-tetrahydro-2-anilinoacridone, as stabilizers for quinacridonequinone pigment.